Summary of Significant Accounting Policies - Summary of Changes in Contract Liabilities (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Contract liabilities:
Deferred revenue, Balance at Beginning of Period	$ 55,932	$ 59,868
Deferred revenue, Additions	1,000	1,000
Deferred revenue, Deductions	(7,842)	(5,474)
Deferred revenue, Balance at End of Period	$ 49,090	$ 55,394